This invention relates to resistivity measurement devices, and more particularly, devices for measuring the resistivity of a surface layer of semiconductor wafers including III-V compound semiconductors.
Most Four-Point-Probe resistivity meters currently available can measure absolute sheet resistivities of Si, Ge or metal films up to 10.sup.6 ohm/sq with good accuracy except for the lightly implanted layers on an opposite type substrate. However, no known commercially available Four-Point-Probe units works on the III-V compound semiconductors at dosages below 10.sup.18 cm.sup.3. This is generally due to the resistance of the contact between the metal point probe and the nondegenerated III-V compound semiconductor being so high that the voltmeter connected between any of the two potential probes of the Four-Point-Probe can not pick up the potential at the probed spot on the semiconductor with less than 0.01% error. This accuracy is required in order to see the small difference of the potentials at the probed spots on the III-V compound semiconductor surface.
For a Four-Point-Probe to make a measurement with less than 1% error, the contact resistance of any of its potential probe to the semiconductor contacts should be less than about 10.sup.-5 times of input resistance of the potential probe. This is due to the potential difference between the two potential probes being as small as 10.sup.-3 times their potential from ground. Therefore, if the input resistance of the potential probe is 10.sup.14 .OMEGA., the contact resistance of the potential probe must be below 10.sup.9 .OMEGA..
The contact resistivity of tungsten to a 2.OMEGA.-cm, N type Si is about 5.times.10.sup.4 .OMEGA./cm.sup.2 at zero bias. This means 5.times.10.sup.9 .OMEGA. for a area of 1000.mu..sup.2 which is a typical metal probe to semiconductor contact area. When the Silicon resistivity is increased 100 times, its contact resistance to a metal is increased roughly one order of magnitude. Therefore, for intrinsic Si which may have a resistivity of about 2.times.10.sup.4 .OMEGA.-cm, the corresponding contact resistance can be about 5.times.10.sup.11 .OMEGA..
Probe pressure and field concentration in the semiconductor near the probe tip may reduce the contact resistance up to two orders of magnitude but some foreign material may be present at the contact interface to increase the contact resistance. In any case, a Four-Point-Probe Meter with the input resistance of 10.sup.14 .OMEGA. at the potential probes can barely be used to measure the resistivity of intrinsic single-crystal Si. The contact resistivity of a metal to a III-V compound semiconductor, such as GaAs, is more than 2 orders of magnitude higher than that to Si, therefore even the more advanced Four-Point-Probe mentioned above has problems in making a clean measurement of GaAs, with the resistivity about a few .OMEGA.-cm. One may, however, reduce the contact resistance by 7 orders of magnitude by applying a 0.5 v forward bias to the contact. Unfortunately, in doing so, the input resistance of the potential probe has to be reduced to one or two orders of magnitude higher than the contact resistance, at its highest, so that the contact can take a large enough bias across it. This is far less than what is desirable, namely 5 orders of magnitude higher than the contact resistance.
It is well known that the technologies and applications of microwave devices, high speed ICs and photovoltaic devices using III-V compound semiconductors as the materials are still in the stages of rapid growth. Obviously, a wider range, faster, more repeatable and more accurate way of measuring the resistivities or sheet resistivities of III-V compound semiconductors for process controls, and for material evaluation would prove helpful to these technologies. The method and circuit of the present invention overcome the high contact resistance problem of probing III-V compound semiconductor wafers and provides an accurate resistivity measurement using a four-point probe or spreading resistance probe.